Complex sentences rely on the precise use of conjunctions to establish logical relationships between clauses, such as cause, contrast, or condition. Analyze the following sentences regarding their structural integrity and the use of connectors:

I. "Despite the weather was cold, they decided to go swimming."
II. "Not only did he finish the project on time, but he also exceeded all quality expectations."
III. "She studied hard for the exam; however, she failed to achieve the necessary grade."

It is correct what is stated